The mode of operation of conventional electric receiving antennas is based essentially on the fact that an incident electromagnetic wave induces a voltage drop across the antenna that constitutes the input signal for the receiving electronics. However, the induced voltage drop is generally so small that it must initially be amplified with the aid of resonance effects. In this case, either the conventional antenna is itself designed in such a way that a geometrical resonance occurs at the receiving frequency, or the antenna is provided with an electric balancing network such that the antenna and balancing network form a resonant circuit.
Owing to the resonance amplification required for achieving an adequate sensitivity, the frequency bandwidth within which reception is possible is severely restricted with conventional antennas. The bandwidth is typically smaller than 20% of the resonance frequency. A larger bandwidth can be achieved with the aid of so-called “frequency independent” logarithmic spiral antennas. However, even here the extent of the antenna must be in the region of the wavelength of the electromagnetic wave to be received, and the sensitivity of such antennas is mostly low.
It is true in principle that magnetic loop antennas can be designed with broader bands than electric antennas, but they cannot be used at relatively high frequencies because of the occurrence of inductive effects and because of the skin effect. Moreover, during conventional operation the impedance of magnetic loop antennas can be adapted to the impedance of conventional electronic circuits only with difficulty, and the efficiency of the antennas is thereby sharply decreased.
A further problem occurs with conventional antenna systems when electromagnetic waves of very low signal strength are to be received. The input sensitivity of conventional antenna systems is limited in principle by the noise of the preamplifier following the antenna. The input signal coming from the antenna must be above the noise threshold of the preamplifier so that it can be usefully amplified. In order to receive very small signals, it is therefore necessary with conventional receiving systems that the antenna extracts a sufficient amount of energy from the electromagnetic field. In the case of very small signal strengths, this can be achieved conventionally only by using very large antennas, parabolic mirrors or arrays of antennas, for example. In any case, the spatial extent of the antenna increases sharply here. A typical application in which small signal strengths are present, is the reception of satellite signals, for example. Here, the extent of the antenna is typically in the region of one meter.
Because of their large dimensions, antenna arrays are not suitable, or suitable only in a severely restricted way, for mobile use, in particular, for example in vehicles, ships or aircraft.
Since conventional antenna systems must extract energy from the electromagnetic field in order to process the information contained in the field, the range of transmission antennas is additionally limited in principle by the number of the receivers for a given transmitter power.